1. Field of the Invention
The present invention relates to an element for generating near-field light, and to a heat-assisted magnetic recording method for performing a magnetic recording in which a magnetic recording medium is irradiated with near-field light, thereby anisotropic magnetic field of the medium is lowered. Further, the present invention relates to a magnetic recording head for writing data by the heat-assisted magnetic recording method.
2. Description of the Related Art
As the recording density of a magnetic disk apparatus becomes higher, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. As the thin-film magnetic head, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic microparticles gathered together. Here, one record bit consists of a plurality of the magnetic microparticles. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic microparticles and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic microparticles. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, write field intensity of the thin-film magnetic head is limited by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. Therefore, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit.
Recently, as a method for solving this problem of thermal stability, so-called a heat-assisted magnetic recording technique is proposed, in which writing is performed by reducing the anisotropic magnetic field with heat supplied to the magnetic recording medium formed of a magnetic material with a large KU just before applying write field. The heat-assisted magnetic recording technique has some similarity to a magneto-optic recording technique. However in the magneto-optic recording technique, the area to which light is applied determines spatial resolution of record bits (that is, a light-dominant technique). Whereas in the heat-assisted magnetic recording technique, the area to which magnetic field is applied determines spatial resolution of record bits (that is, a magnetic-field-dominant technique).
As a heat-assisted magnetic recording technique, a method has been generally known, in which a near-field light probe formed of a metal piece, so-called a plasmon antenna, is used for generating near-field light from plasmon that is excited by irradiated laser light. For example, U.S. Pat. No. 6,768,556 B1 discloses a plasmon antenna that includes a metal scatterer with a strobilus shape formed on a substrate and a dielectric material film formed around the metal scatterer. And US Patent Publication No. 2004/081031 A1 discloses a configuration in which a plasmon antenna is formed in contact with the main magnetic pole of a magnetic head for perpendicular magnetic recording in such a way that the irradiated surface of the plasmon antenna is perpendicular to the surface of a magnetic recording medium. Further, US Patent Publication No. 2003/066944 A1 discloses a technique in which the tip of a plasmon antenna is made closer to a magnetic recording medium to attempt to irradiate the medium with stronger near-field light.
However, a difficult problem may arise as described below in achieving the heat-assisted magnetic recording by using a plasmon antenna as a near-field light generating part.
It is generally known that, while the plasmon antenna converts the laser light irradiating the plasmon antenna into near-field light as described above, its light use efficiency is around 10% at the highest. That is, most of the irradiating laser light is changed into thermal energy within the plasmon antenna except the light component reflecting off the surface of the plasmon antenna. Here, the size of the plasmon antenna is set to the wavelength of the laser light or less, and its volume is very small. Therefore, the plasmon antenna is brought into a very high temperature due to the thermal energy. For example, a simulation result has been obtained, in which, when a plasmon antenna, which is a flat plate formed of Au and having an equilateral triangle shape with each edge of 300 nm (nanometers) and a thickness of 50 nm, absorbs the laser light of 17 mW at room temperature, its temperature rises to 500° C. (degrees centigrade).
Such temperature rise causes the plasmon antenna to thermally expand and to protrude from an opposed-to-medium surface toward the magnetic recording medium. Resultantly, such a situation is possible to occur that the end of a read head element reaching the opposed-to-medium surface relatively moves away from the magnetic recording medium, the read head element provided for reading data signals or servo signals from the magnetic recording medium. In this case, during write operation in which the magnetic recording medium is irradiated with the near-field light by using the plasmon antenna, it becomes difficult to read the servo signals well.
It is conceivable to compensate such protrusion of the plasmon antenna (and the end of a write head element) by protruding the end of the read head element with heat generated from a heating resistor provided in advance, but it is difficult to put the compensation into practice. Actually, while a response time from the irradiation of laser light to the thermal expansion of the plasmon antenna is of the order of 10 μsec (microseconds), a response time from power-on of the heating resistor to thermal expansion of the end of the read head element is of the order of 100 μsec, that is, one digit larger. Resultantly, it is very difficult for the end of the read head element to protrude without delay and follow the thermal expansion of the plasmon antenna during write operation.
Moreover, under this very high temperature of the plasmon antenna, the electric resistance of the plasmon antenna becomes significantly high. That is, thermal disturbance of free electrons in the plasmon antenna becomes large, which may cause further degradation of the aforementioned light use efficiency of the plasmon antenna.
Another problem may arise when the plasmon antenna is used in combination with a magnetic pole of the write head element. Practically, when the heat-assisted magnetic recording is carried out, it is necessary to provide the plasmon antenna and the magnetic pole so as to be positioned as close as possible to each other. Particularly, it is preferable to set a distance between both parts to 100 nm or less. With this arrangement, the field gradient of write field given from the magnetic pole can be made sufficiently large at the position irradiated by the near-field light on the magnetic recording medium. In this case, however, as disclosed in US Patent Publication No. 2004/081031 A1, the laser light to irradiate the plasmon antenna also passes through positions spaced apart from the magnetic pole by an extremely smaller distance than its own wavelength. Resultantly, a portion of the laser light is absorbed into the magnetic pole made of metal, which may reduce the amount of light to irradiate the plasmon antenna.